He shot a security officer in the head, killing him, and took his 9 mm pistol and a magazine of ammunition. The shooter then continued through the building and seemed to target his victims, who were mostly on the third and fourth floors, Bensen reported.

D.C.’s Metropolitan Police Department and several other law enforcement agencies responded with active shooter teams.

During that response, a MPD officer was shot in the leg. The gunman was then shot by a FBI hostage response team, law enforcement sources told Bensen.

According to what witnesses are telling investigators, by the time the shootings ended, the gunman was seen with a semiautomatic 9 mm pistol and an AR-15 assault rifle. Authorities are investigating whether the gunman took the D.C. police officers’ rifle.

This report states that the shooter, Aaron Alexis, was armed only with a shotgun as he entered the building. He then ambushed first responders and took their weapons and ammunition, creating his arsenal by acquisition as he carried out his attack.

If this account is true, there is a chance that the rifle in question was not an AR-15, but a selective-fire M4 or M16.

If that proves to be the case, then this would be the first time in the history of the United States that a legal (until it was taken), selective-fire assault rifle was used in a violent crime.

Update: A national NBC News report confirms that Alexis collected the rifle and pistol used in the shooting, but slightly changes the story to claim that the rifle was taken from a gun safe:

He packed an assault rifle and two other firearms during his lethal rampage — including a shotgun purchased just weeks ago at a Virginia gun shop, multiple law enforcement officials told NBC News.
The officials also said Alexis obtained the assault rifle used in the shooting from a gun safe on the naval base. There’s where he may have also obtained a handgun, according to authorities.
One official who spoke to NBC News, however, cautioned there are still outstanding questions about where and how Alexis acquired the weapons used in the massacre.

Are Navy Shore Police, NCIS agents, Marines, or other security personnel at the Navy Yard in the habit of leaving gun safes open?